Ayanamsha is the angle between two different ways of deciding where the zodiac begins.
The tropical zodiac starts at the March equinox — the point where the Sun crosses the celestial equator going north. The sidereal zodiac starts at a fixed position among the stars. Because the Earth's rotational axis precesses, completing one slow circuit in about 25,800 years, the equinox point slides backwards along the ecliptic at roughly 50 arcseconds per year. The gap between the two zodiacs therefore grows, and that gap is the ayanamsha. It is currently about 24°.
Jyotish is sidereal, so every Indian chart needs an ayanamsha value to convert from the tropical positions an ephemeris naturally produces. And here is the difficulty: there is no agreed answer to which star, at which epoch, defines sidereal zero. Lahiri (also called Chitrapaksha) fixes Spica at exactly 180°, and is what the Indian government's Calendar Reform Committee adopted in 1955 and what the national ephemeris uses. Raman, Krishnamurti, Yukteshwar and Fagan–Bradley all differ, by amounts ranging from a fraction of a degree to over two degrees.
Two degrees is enough to move a graha near a sign boundary into the next sign, and enough to shift a nakshatra pada — which changes the navamsha and the dasha balance. When two astrologers disagree about a chart, the ayanamsha is the first thing to check, before anything interpretive. This site uses Lahiri, and says so on every calculation, because a computed chart with an unstated ayanamsha is not reproducible.